It can be over in fifteen minutes. It can last several hours. It can be done with a Wurlitzer - or twenty guitars. Now In C, the defining work of minimalist music has been tackled by Damon Albarn and Africa Express. Its composer Terry Riley reveals how it all began.

Terry Riley is sitting in a quiet corner of the legendary Café Einstein in Berlin's red-light district. A patient smile plays around his face - but mention the m-word and irritation suddenly ripples through his great candyfloss beard. "Minimalism was never a word we used for what we did," he says. "It was a tag from the art world someone stuck to us later. My heart sinks when I get emails from music students saying they are writing a 'minimalist piece'. Once you become an ism, what you're doing is dead."

Riley's problem is that he is the composer of perhaps the defining work of what has become known as minimalist music. In C, the groundbreaking piece he first performed on November 4, 1964, may have become an "ism" but it is no artefact: for its fiftieth anniversary, tributes and reinterpretations keep coming. A new version - recorded by Damon Albarn and his Africa Express team, with help from Brian Eno and Nick Zinner of the Yeah Yeah Yeahs - is out this week. Today, the Californian is in Berlin to meet with the German conductor André de Ridder, who instigated and led this new version, recorded last year in Bamako, the capital of Mali. For his eightieth birthday next year, there will be Riley retrospectives at music festivals around the world. The beard will be doing a lot of rippling.

In C was born in the blue fog of Parisian jazz. In February 1962, Riley and his wife arrived in the French capital from the US. He made a living playing boogie-woogie piano at military bases and at Fred Payne's Artists Bar in Pigalle. In the pool room of the bar one day, he bumped into jazz legend Chet Baker, who had just returned from Italy where he had been imprisoned for drug possession.

The two hit it off and, in June 1963, were asked by the playwright Ken Dewey to come up with a soundtrack for an experimental theatre piece called The Gift. Riley recorded Baker and his band playing a number of works, including Miles Davis's So What, and then starting messing about with the tapes in the studio. "I added a spoken-word part, created loops with Chet's trumpet, and let them run out of sync. It was a kind of precursor to In C."

The piece itself came to Riley about eleven months later, when he was back in the US. "I was on a coach in San Francisco," he says, "and it just revealed itself to me, like in a dream." By the end of the next day, he had written In C down on paper, and a few months later the piece premiered at the San Francisco Tape Music Center. Another Riley work performed at that November 4 concert was Coule, its title riffing on the French verb "to flow" and its English homophone. But In C didn't flow in the way cool jazz did: it had an incessant, throbbing forward drive. To modern ears, it sounds a bit like the string section from the Psycho shower scene played by a bunch of stoned beatniks, or the London Philharmonic doing krautrock.

With In C, Riley wanted to achieve the same effect with a live orchestra that he had previously created electronically: the score has fifty-three different musical phrases, lasting from half a beat to thirty-two. Performers can play these phrases over and over, starting them at different times, so the music falls in and out of sync. A performance can be over in fifteen minutes, or last several hours, with a steady pulse of the note C - played in the original performance by Steve Reich on a Wurlitzer organ - keeping chaos at bay. The first gig featured fifteen musicians, though Riley now thinks twenty-five to thirty are needed "to build up the right wall of sound". Last October, Portishead's Adrian Utley recorded it with nineteen other guitarists.

Riley isn't too fond of talking about the work these days, though. "I wanted to keep on doing something new," he says. A few weeks after that first performance, he recalls, Steve Reich started publishing his own sound collages and people started saying to Riley: "Oh, what you're doing sounds a lot like Steve's work." As Riley says: "This wasn't easy for me to take. I felt it had taken me years to find a style that gave me an identity, and now it was suddenly being taken away from me."

With 1969's A Rainbow In Curved Air, he returned to electronic overdubs, and his current compositions are more traditionally classical, albeit with gospel and jazz elements. "I am still interested in music as ecstasy, as something that transports you away from the everyday to another place." But he was excited when De Ridder sent him a Soundcloud link to the Africa Express version. He immediately emailed him back, saying it sounded as if his piece was "taking flight with the soul of Africa".

While the term minimalism evokes disciplinarians in black rollneck jumpers, the enduring spirit of In C is anything but: it snatches the baton from the composer and hands it to the musicians. Riley recalls recording a version with the Shanghai Film Orchestra in the 1980s. "They wanted to use all these western instruments because they couldn't reach the fourth degree of the scale on their instruments. But that's precisely what I loved about it."

The Africa Express version is, likewise, faithful to the spirit rather than the letter of In C. De Ridder had brought a copy of Riley's score to Bamako because he had been performing the piece with his own ensemble, Stargaze. He soon found out that straight repetition was alien to the philosophy of Malian musicians. Instead, they ended up jamming over him playing the phrases on the violin.

Riley's favourite part of their version, he says, is when the C pulse suddenly stops and one of the musicians starts speaking over the music, about the first time he learnt to play his instrument, the kora. "That blew my mind," he says. "I always welcome it when pieces change. The worst interpretations I've heard of In C mechanically try to copy the original performance." The thought sends one final quiver of irritation through his facial hair. "Rules," he says, "are not as important as results."